The present invention relates generally to performance improvement of induction type electric machines and, more particularly, to structure and geometry of the end ring of an induction rotor being operated at high speed and high temperature relative to the material properties of the end ring.
An induction motor is an asynchronous electric machine powered by alternating current (AC), where such power is induced in a rotor via electromagnetic induction. For example, polyphase AC currents may be provided to stator windings structured to create a rotating magnetic field that induces current in conductors of a rotor, whereby interaction between such induced currents and the magnetic fields causes the rotor to rotate. Induction motors may have any number of phases. An induction motor may operate as a generator or traction motor, for example when driven at a negative slip.
Rotors of induction motors may conventionally include a cage such as a squirrel cage having axially parallel or skewed conductor bars of copper or aluminum extending between opposite rotor ends and positioned at radially outward locations along the circumference of the rotor. Distal ends of individual conductor bars may be provided with structural support and be in electrical communication with one another by connection of the respective bar ends to one or more continuous end rings disposed at each rotor end. The rotor may have a substantially cylindrical iron core formed as a stack of individual laminated disks of a silicon steel material. Each core disk may have axial slots for passing the copper or aluminum bars therethrough when the slots are in alignment with one another.
Due to the high costs associated with permanent magnet electric motors, electric machines for many different applications are being redesigned to utilize induction rotors. However, conventional die-cast induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen die-cast material, especially when structural weakness is exacerbated by the size and speed of the rotor. Conventional induction machines may utilize varying grades of aluminum or copper in die-casting the end rings/plates and the conductor bars of the cage as an integral unit. Depending on the grade, the cast material strength may vary significantly. Such variation may be the result of achieving other material properties, for example those related to thermal conductivity, electrical conductivity, suitability for die-casting, etc.
When an induction motor is utilized in a given application such as automotive, the rotor must tolerate high speed rotation and associated large centrifugal force. In addition, high temperatures, potential metal fatigue, and other factors may aggregate with forces acting in a radial outward direction and those acting in an axial direction to cause structural breakdown resulting in damage or deformation of the cast end rings of a rotor. For example, an induction rotor generates higher temperatures within the rotor itself, further reducing mechanical and structural integrity of end rings.